Fix the usage of parallel and full options of vacuum command.

 Fix the usage of parallel and full options of vacuum command.
 
 Earlier we were inconsistent in allowing the usage of parallel and
 full options.  Change it such that we disallow them only when they are
 combined in a way that we don't support.
 
 In passing, improve the comments in some of the existing tests of parallel
 vacuum.
 
 Reported-by: Tushar Ahuja
 Author: Justin Pryzby, Amit Kapila
 Reviewed-by: Sawada Masahiko, Michael Paquier, Mahendra Singh Thalor and
 Amit Kapila
 Discussion: https://postgr.es/m/58c8d171-e665-6fa3-a9d3-d9423b694dae%40enterprisedb.com